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post #1 of 10 (permalink) Old 11-15-2007, 02:46 PM Thread Starter
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Exhaust Question

ok so I am planning on getting my exhaust done here soon, I am getting all new high flow cats, 2.25" pipes and magnaflow mufflers, Although I am considering deleting the third cat but I don't know if it would cause emissions issues. No resonator of course as I am told that it just makes back pressure. My problem is I was told by a shop that these cars came from the factory with 2.25" pipes. Now I am not a certified mechanic but I do know a few things. That just sounds like BS to me. But that does make me interested in what the stock size was. Anyone know? Also let me know what you think about my exhaust before I get it, wanna make sure I am getting the best set up.

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post #2 of 10 (permalink) Old 11-15-2007, 03:39 PM
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I believe that the stock size is 2.25" before going into a single 2.5" pipe. Removal of the third cat won't cause you to fail emissions unless your engine isn't running right and it was taking up the slack of the first 2. I passed smog in CA with only the front 2, and the readings for HC, CO, and NO were almost zero.

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post #3 of 10 (permalink) Old 11-15-2007, 06:51 PM Thread Starter
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so does that mean I can just reuse the stock pipes and throw on the high flow cats and mufflers and just delete the resonator and get the same performance? If so that would be much cheaper.

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post #4 of 10 (permalink) Old 11-15-2007, 07:38 PM
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You can probably do that but I've never seen the exhaust of a 5.0L MN12. With the 4.6L, new downtubes will yield benefits as the stock collector angle isn't the most conducive to unimpeded flow.

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post #5 of 10 (permalink) Old 11-17-2007, 06:33 PM
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You'll want to replace the downtubes if they're factory. If I remember correctly, the driver's side is especially bad. On mine the factory downtube from the manifold dropped almost straight down and entered the cat from the top, and exits out the rear. A 90° turn in only a few inches. Quite a restriction. Here's a pic of a 3.8 SC exhaust - the driver's side is similar to what came from the factory on mine:

mn12-techinfo-exhaust


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post #6 of 10 (permalink) Old 11-17-2007, 11:28 PM
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these cars have no bigger than a single 2.25 pipe from factory

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post #7 of 10 (permalink) Old 11-18-2007, 03:50 AM Thread Starter
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ok so then simplest and cheapest option is to replace the stock downtubes, replace the cats, delete resonator and add new mufflers? And just leave the stock pipes?

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post #8 of 10 (permalink) Old 11-18-2007, 04:14 AM
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FWI the resonator creates no backpressure i have seen thru a stock mn12 resonator and you can see strait thru it. Its only to reduce sound. fwi backpresure is not necesarily good actually its bad. velocity is what matters a pipe too big will have slower velocity killing the vaccum effect on pulling the next spent charge along.

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post #9 of 10 (permalink) Old 11-19-2007, 04:17 PM Thread Starter
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I have been told by three different shops that resonators do create backpressure, just not much to worry about. I just figure get rid of as much backpressure as possible. I am still looking into the possibility of deleting the third cat. I will have to check with some smog shops I guess.

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post #10 of 10 (permalink) Old 11-19-2007, 07:27 PM
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Well they do create a little back pressure. It creates turbulence when the exhaust expands from the reg pipe into the chamber, then has to funnel back into the exit pipe.

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